The Uminaoshi are a type of mushi - a mass of glowing tendrils which feed on the time a person has been alive. They then send the person back to an embryonic stage so they can be reborn. They are found in underwater trenches, and don't come out when the moon is visible. They glow in the dark in order to attract and capture prey.
The embryo released by the Mushi
When it is a full moon, they release the embryonic eggs back to the water surface. If a human woman eats an orb containing a human embryo, they become pregnant with them, and they are born, completely and genetically identical to the Mushi’s victim. When a human is reborn this way, they grow up just as they did last time, retaining habits and mannerisms of their previous life. There is an island village whose villagers use this mushi to live a pseudo-eternal life. They will take the bodies of their dead to the Uminaoshi and use the resulting embryo egg to have one of their women give birth to that person again. This means that a daughter could give birth to their own mother, making that person both their parent and child.